Measurement of the cross sectional area of a body cavity such as a blood filled ventricle of a heart is useful for monitoring its operation, and measurement of its volume is useful in determining physiological factors such as stroke volume, cardiac output and the ejection fraction. At present, measurements of cross sectional area and volume are not attainable on a real time basis without the use of invasive procedures such as catheterization techniques in which foreign substances are injected into the bloodstream. Because of the risks involved, these procedures are not used to monitor patients on a continuing basis.
Off line non-invasive methods for measuring a cross sectional area of a ventricle are known, but they are laborious and time consuming. In one, the cross sectional area of a ventricle appearing in successive frames by an ultrasonic imaging apparatus is measured by hand. In another method, the edge of a cross sectional area in one frame is drawn by hand and optical flow techniques are used to identify the location of the edge in successive frames. Whereas this outlines the cross sectional areas more rapidly than can be done by hand, the areas are still measured manually.
In employing the optical flow techniques it is necessary to provide a tissue indicator for indicating where the edges of a ventricle occur. A circuit for this purpose is described in an article entitled Automatic Real Time Endocardial Edge Detection In Two Dimensional Endiocardiography at pages 303-307 of Ultrasonic Imaging 5, 1983, and incorporated herein by reference. In this circuit a reference voltage having a value between the higher amplitude expected of image signals caused by reflections from tissue and the lower amplitude expected of image signals caused by reflection from blood is applied to one input of a comparator, and the image signals are applied to the other. Thus, the output of the comparator may be 1 for signals derived from reflections from tissue and 0 for signals derived from reflections from blood so that a change from 1 to 0 or from 0 to 1 indicates the presence of an edge. A one bit filter such as described in the article noted below that relates to rational gain control is connected to the output of the comparator so as to provide a signal that changes state only when the output of the comparator has changed state for a given length of time.
There is another problem however. As is well known, the amplitude of the reflections from structures within the body decreases with range because of absorption of the energy from the transmitted pressure waves as they pass through the body. Thus it is possible for the amplitude of the image signals due to reflections from tissue on the far side of the ventricle to become less than the reference voltage referred to above and thereby fail to cause the change in the comparator output that would indicate the location of the far edge. For this reason, it has been customary to gradually increase the gain of the amplifier to which the image signals are applied after each pulse is transmitted. It has been suggested that improved results can be obtained by using a rational gain control circuit, RGC, that increases the gain of the amplifier more rapidly when the comparator of the edge detector circuit indicates that the image signals are due to reflections from tissue than when it indicates that the reflections are from blood. Such a circuit is described in an article entitled Rational Gain Compensation For Attenuation In Cardiac Ultrasonography at pages 214-228 of Ultrasonic Imaging 5, 1983, and incorporated herein by reference. Unfortunately, however, noise spikes can still cause the comparator to change its output so as to give a false indication of the position of a edge.